1. Field of the Invention
The present invention generally relates to a reduced size high frequency quadrupole for accelerating ions in a neutralized ion beam.
2. Description of the Background Art
Radio frequency quadrupoles (RFQs) have been used for the confinement and acceleration of ion particles for many years. RFQs generally consist of four elongated electrode rods surrounding a central axis through which an ion beam passes. The electrodes are driven by an oscillating voltage to produce an electromagnetic field. Such radio frequencies produced can be any periodic, time varying waves such as, UHF or microwaves. When the frequencies are applied, rods opposite one another assume opposite charges thereby establishing an electric field having a certain direction between the electrode rods.
Transverse confinement of positive and negative ions occurs when the RFQ structures are in a cylindrical geometry. When the periodic, time varying voltage is applied between the pairs of quadrupole rods, particles having charge-to-mass ratios in a certain range (dictated by the quadrupole dimensions, driving voltage, and driving frequency) are confined transversely while remaining free to move longitudinally. This principle is employed in quadrupole mass spectrometry. The confinement occurs regardless of the sign of the particle charge.
Acceleration of the particles in the longitudinal direction is produced by perturbing the shape of the surface of the quadrupole rods facing the central axis. When using a microwave frequency quadrupole (MFQ), a sinusoidal scallop is added along the axis of the vanes or electrodes. While maintaining the radial trapping, this structure imposes a longitudinal traveling wave that moves down the axis at a speed governed by the driving frequency and scallop wavelength. Particles are trapped in bunches by the wave, and are accelerated as the scallop wavelength increases. So far, MFQs have been used only to accelerate single species of particles.
When positive and negative ions of the same charge-to-mass ratio are combined in a beam in equal proportions, the beam has overall charge neutrality, and is called a “neutralized ion beam.” Neutralized ion beams will avoid charging the target; a charged target can cause repulsion of the incident ion beam, thereby requiring higher and higher energies of the incident beam for penetration. Unlike in an ion-electron neutralized beam, the positive and negative species of a neutralized ion beam respond symmetrically to electric and magnetic forces. This property lends itself to a range of applications, including fusion energy research, plasma processing, and ion propulsion, as well as to the acceleration of the beam itself. Currently, fusion devices employ neutral beam injectors that are very large in size and/or cannot efficiently produce MeV beams. What is needed is a reduced size compact and efficient RFQ which provides a neutralized ion beam of high energy.